The present invention relates to a method for producing non-ageing cold rolled steel sheets having a noticeably excellent formability.
Outer panels and inner panels of automobiles are subjected to high press forming, so that non-ageing cold rolled steel sheets having a high r value and a large elongation have been used. In particular, for fenders, quarter panels and oil pans have been used decarburized and denitrogenized steel sheets produced through open coil annealing and Ti killed extra low-carbon steel sheets but the former is high in the production cost and large in the grain size and low in the strength, so that upon press forming, skin roughness referred to as orange peel and wall break are apt to be caused. Furthermore, in the latter Ti killed steel sheets, Ti has a strong bonding force to not only C and N but also S and O, so that in order to ensure the non-ageing property, Ti must be added in an amount of several times of the stoichiometric equivalent with respect to C and N and the formed titanium sulfide and oxide become non-metallic inclusions and a large number of surface defects referred to as sleever are formed.
As an improved method, it has been proposed that C and N are fixed by Nb or Nb and Al, instead of Ti and the ageing property and r value are improved, and for example, Met, Trans. 1972, vol. 3, pp. 2171-2182 discloses that when Mn and Al are compounded in sufficient amounts to fix S and N respectively in steel, if Nb is contained in a sufficient amount to form NbC stoichiometrically with respect to C, that is 0.071% of Nb per 0.009% of C, non-ageing steel can be obtained by annealing at 700.degree. C. within 4 hours after cold rolling and if 0.177% of Nb per 0.014% of C is contained, even when annealing is effected at 870.degree. C. after cold rolling, no yield elongation occurs.
U.S. Pat. No. 3,761,324 discloses that when 0.068-0.25% of Nb is added to steel containing 0.002-0.20% of C and Mn in a sufficient amount to S, if Nb which is not bonded to C and N, is present in an amount of more than 0.025%, r value reaches more than 1.8. Examples in this patent show that non-ageing steel sheets having a r value of 1.78-2.10 and an elongation of 40-48% can be obtained by hot rolling and cold rolling a material containing 0.005-0.01% of C, about 0.006% of N, 0.015-0.020% of Al and 0.08-0.12% of Nb and then annealing the thus treated sheet at 700.degree. C. for 1-16 hours.
Other than the above described methods, a plurality of methods wherein Nb alone is added without using Al, have been proposed but any method naturally needs a larger amount of Nb than the above described two methods in order to obtain non-ageing steel sheets and the formability is poor.
Thus, the previously proposed Nb added steels are non-ageing cold rolled steel sheets having a r value of 1.6-2.1 and an elongation of 40-48% obtained by adding 0.07-0.18%, preferably 0.08-0.12% of Nb to extra low-carbon steel containing 0.005-0.02% of C but have the following defects.
(1) These steel sheets have an elongation of 40-48%, which is lower than the elongation of 50-54% of decarburized and denitrogenized steel sheets. PA1 (2) Nb is an expensive metal and the cost is necessarily increased by the addition of a large amount of Nb.
An object of the present invention is to provide a method for producing non-ageing cold rolled steel sheets, in which the drawbacks of the previously known methods are obviated and improved.
In order to prolong the durable life of thin steel sheets used for outer panels of automobiles, the demand of steel sheets on which a coating is applied, has been increased. For this coating, a variety of processes have been developed but in view of the production cost and the properties thereof, a continuous hot-dip galvanizing process is one of the most excellent processes.
Since the outer panels and inner panels of automobiles are subjected to the high press forming, non-ageing galvanized steel sheets having a high r value and a large elongation are necessary. The method for producing the galvanized steel sheets to be used for such an object involves (1) a method wherein cold rolled low-carbon steel sheets are plated by usual step, that is continuous annealing-continuous plating and then subjected to over ageing treatment to obtain non-ageing steel sheets (Japanese Patent Application Publication No. 74/72), (2) a method wherein carbide-forming elements which do not retard the plating ability, such as Nb are added to a steel material in addition to Ti, whereby non-ageing steel sheets are obtained (Japanese Patent Laid-Open Application No. 35,616/78) and the like but these methods cannot provide the satisfactory drawing property, that is high elongation high and r value.
Another object of the present invention is to provide a method for producing galvanized steel sheets having a very high r value and a high elongation, an excellent deep drawing property, that is substantially non-ageing property and an excellent surface property.